Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, such packages are made on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled from the top with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by two pairs of jaws. More specifically, the pairs of jaws act cyclically and successively on the tube to seal the packaging material of the tube and form a continuous strip of pillow packs connected to one another by respective transversal sealing bands.
The pillow packs are separated by cutting the relative sealing bands, and are then conveyed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
In a first alternative, the packaging machine comprises two forming assemblies movable along respective guides and which interact cyclically with the tube to heat seal the packaging material of the tube. Each forming assembly comprises a slide movable back and forth along respective guide; and the two jaws which are hinged at the bottom to the respective slide, and move between a closed position, in which they cooperate with the tube to seal it, and an open position in which they are detached from the tube.
In a second alternative, the packaging machine comprises two chain conveyors defining respective endless path and fitted with respective number of jaws. The two paths have respective branches substantially facing and parallel to each other, and between which the tube of packaging material is fed so that the jaws on one conveyor cooperate with the corresponding jaws on the other conveyor along branches of respective paths, to grip the tube at a number of successive cross sections, and to seal and cut the packs.
In the case of aseptic packages with an aluminium layer as the barrier material, the cross sections of the tube are normally sealed by induction of loss current in the aluminium layer to melt the heat-seal plastic material locally.
More specifically, one of the jaws in each pair comprises a main body made of nonconductive material, and an inductor housed in a face seat on the main body; while the other jaw has pressure pads made of flexible material, such as rubber.
The inductor is energized when the relative pair of jaws grips the tube, so as to seal a cross section of the tube by heat sealing the plastic covering material.
In the case of packages with no layer of aluminium or other electrically conductive materials, the cross sections of the tube are normally sealed using a hot plate for locally heating the packaging material inwards.
More specifically, one of the jaws is equipped with a hot plate, while the other normally has one or more pressure pads of flexible material. This method, commonly referred to as “hot-plate” sealing, calls for relatively prolonged contact between the hot plate and the packaging material.
To accelerate local melting of the packaging material, and so increase package production speed, increasing use is made of ultrasonic sealing devices, which substantially comprise a mechanical-vibration generator or sonotrode and an anvil—known for example from EP-B-615907 in the name of the same Applicant—fitted to respective jaws in each pair and which cooperate with each other to heat the packaging material by means of ultrasonic vibrations.
More precisely, the sonotrode disclosed in EP-B-615907 substantially comprises a head and a unique drive unit which consists of a stack of alternate piezoelectric ceramic plates and conductive metal sheets.
The head defines a sealing surface which extends along a first direction, and the drive unit is fitted to the head on the opposite side of the sealing surface.
The drive unit is coupled to an alternate current generator. In this way, the piezoelectric ceramic plates convert the electrical supplied voltage into a mechanical strain which causes the oscillation of the head and, therefore, the sealing of the transversal sealing band.
In particular, the sonotrode constitutes a half-wave resonator, i.e. the entire length of the sonotrode along a second direction orthogonal to the first direction consists of half a wave length of the oscillation of the head.
Ultrasonic sealing devices of the type described in EP-B-615907 have the following drawback.
When a longer transversal sealing band is desired, a plurality of the above-described sonotrodes must be interconnected for an additional length.
As a consequence, when the package of the volume of about 200 ml must be transversally sealed, two sonotrodes must be interconnected to each other.
Furthermore, the inevitable errors during the interconnection of the sonotrodes are likely to generate a non-uniform vibration between such sonotrodes. Accordingly, the resulting transversal sealing band may be uneven.
To overcome this drawback, WO-A-2007/020208, in the name of the same Applicant, discloses a single-piece sonotrode which accommodates three driven units fed by a single generator. In greater detail, the three drive units are fixed into respective recesses of the sonotrode, and have respective plurality of piezo-elements.
However, the sonotrode disclosed in WO-A-2007/020208 is not optimized for transversally sealing packages of about 200 ml, because the sealing surface resulting by the presence of three drive units is larger than required. For the same reason, the sonotrode disclosed in WO-A-2007/020208 cannot replace two adjacent sonotrodes of the type shown in EP-B-615907 in an existing packaging machine. As a matter of fact, the sealing surface of the sonotrode disclosed in WO-A-2007/020208 is larger than the sum of the length of two sealing surfaces of respective adjacent sonotrodes of the type shown in EP-B-615907.
Finally, the sonotrode disclosed in WO-A-2007/020208 comprises a pair of slots which are arranged between respective couples of drive units adjacent to one another.
The sonotrode also comprises a plurality, in particular three pairs, of S-shaped hooks for fixing the sonotrode to the frame of the jaw. The S-shaped hooks of each pair are arranged on opposite lateral side surfaces of the sonotrode and may be welded or unitarily formed to the sonotrode. Accordingly, the sonotrode is prevented from oscillating at the S-shaped hooks. In other words, the S-shaped hooks define respective nodal points of the sonotrode and lie on a “so-called” nodal plane of such sonotrode.
More precisely, slots extend orthogonal to a sealing surface of the sonotrode and between two subsequent pairs of S-shaped hooks. In particular, slots extend both above and below the S-shaped hooks. Accordingly, slots pass through the nodal plane of the sonotrode.
The Applicant has found that the vibration of the sonotrode disclosed WO-A-2007/020208 is made unstable by some additional parasitic frequencies.
Accordingly, the resulting transversal sealing band may still be not completely uniform and continuous. Furthermore, these additional parasitic frequencies result in additional fatigue stresses onto the sonotrode, with a consequent reduction of its fatigue residual life.
In order to reduce as far as possible the above-mentioned additional parasitic frequencies, WO-A-2011/117119, in the name of the same Applicant, discloses a sonotrode in which the head continues to accommodate three drive units, but the slots do not cross the nodal plane.
In other words, the sonotrode comprises a pair of first slots arranged on a first side of the nodal plane and a pair of second slots arranged on a second side of the nodal plane.
Furthermore, both the first slots and the second slots extend substantially parallel to a symmetry plane of the sonotrode and, therefore, orthogonally to the sealing surface of the sonotrode.
For the same reasons of the one disclosed in WO-A-2007/020208, the sonotrode disclosed in WO-A-2011/117119 is not optimized for transversally sealing packages having a volume of about 200 ml and cannot be retro-fitted in the existing packaging machines.
A need is therefore felt to render the above-identified sonotrodes optimized for transversally sealing packages of about 200 ml and suitable for being retro-fitted in existing packaging machines in place of two pairs of adjacent sonotrodes shown in EP-B-615907.
Furthermore, though highly reliable and efficient, the sonotrode disclosed in WO-A-2011/117119 leaves room for further improvement.
In particular, a need is felt within the industry to render the amplitude distribution of the vibration of the above-identified as even as possible, in order to increase the quality of the transversal sealing along the length of the same transversal sealing.
For the same reasons, a need is felt within the industry to render the above-identified amplitude distribution as stable as possible, when a non-uniform load is applied along the sealing surface.
For the same reasons, a need is also felt within the industry to increase the distances between the parasitic frequencies in comparison with the known solutions.